Thermostatic switch



g- 1950 T. N. FLIGHT ETAL 2,51

THERMOSTATIC SWITCH Filed Dec. 9, 1948 2 Sheets-Sheet 1 Aug. 22, 1950 T.N. FLIGHT ETAL THERMOSTATIC SWITCH Filed Dec. 9, 1948 2 Sheets-Sheet 2mu e/1734s Mar a I) a Patented Aug. 22, 1950 UNITED STATES PATENT OFFICETHERMOSTATIC SWITCH company Application December 9', 1948, Serial No.64,326 In- Great Britain December 9, 1947 3 Claims. 1

This invention relates to thermostats embodying a magnetic snap actionswitch and in which the thermally sensitive member actuating the switchcomprises a tube or rod of relatively high thermal expansion materialassociated with a rod or'tube of low coeificient of expansion, and moreparticularly to thermostatic switches embodying a magnetic snap actionswitch of the type described in the specification of our copendingapplication No. 39,276, filed July-17, 1948.

One use of thermostats of this type is to control electric waterheaters, and in such apparatus the space available for the switch headof the thermostat is very limited. It is also required that thetemperature adjustment provided on the thermostat be in the form of arotatable knob in the switch head located concentrically with the stemof the instrument.

It is the object of this invention to embody an improved magnetic snapaction switch of the type referred to within a switch head of minimumdimensions and to provide adjustment for the temperature settingconcentric with the stem.

As disclosed in the specification of application No. 39,276 the magneticsnap action switch of this invention consists of a resilient switchmember carrying two iron armatures positioned one on each side of apermanent magnet, and so adjusted for movement that one armature remainscloser to the magnet than the other in both the open and closedpositions of the switch. This adjustment of armatures produces amagnetic bias acting as a closing force between switch contacts, one ofwhich is carried by the resilient switch member, the co-acting contactbeing arranged with screw adjustment in a supporting plate.

By the use of this arrangement of magnet and twin armatures the forceneeded for switch operation is limited to the adjusted difference inpull of the magnet on the two armatures, which force can be of such lowvalue relative to the strength of the resilient switch member thatconsistent and reliable action can be obtained within a switch head ofminimum dimensions.

Thermal movement consequent on a temperature change of the stem of thethermostat is transmitted to the resilient switch member through anon-resilient lever rigidlyisupported at one end and pressing the switchmember at its opposite end, the movement being applied at approximatelythe centre of the non-resilient lever member to press the switch to openposition with a rising temperature.

In the accompanying drawings Figure 1 is 'apart section in elevation onthe line I-I'of Fig. 2

2 showing a thermostat constructed in accordance with this invention,Fig. 2 is a plan view with the casing broken away and Fig. 3 anisometric view of the resilient switch lever with its attached armaturesand contact. Figures 4 and 5 show an elevation and plan of anotherembodiment of this invention. Similar parts in Figures 1, 2, 4 and 5 areindicated by the same reference numerals.

Referring to the drawings, in the base I is fixed a thermally sensitivestem comprising a tube 2 of brass or similar metal of high thermalexpansion into which is screwed at the end remote from the base i anickel iron rod 3, having a low expansion coefficient. Clamping posts 4,i screwed into the base i carry spacing washers 5, between which washersare held the magnet support plate 6, the resilient switch lever l, andfixed contact plate 8.

Electrical insulation is provided on the clamping posts by steatitebushes (not shown), and mica washers as at 5, provide the insulationbetween the switch parts. Nuts it, til, screwing on to the end of theclamping posts i, d clamp the switch pack rigidly to base I. A permanentmagnet ll fixed into its carrying plate 6, is embraced by a twinarmature unit l2, #3 attached to the centre limb of the resilient switcharm I, by riveting with the shank of the moving contact i l. Fixedcontact [5 is screwed into plate 8.

The position of the armatures i2, i3 relative to magnet H, and thetravel of the moving contact [4 is set by adjustment of contact i5 andback stop 2 I.

Figure 4 shows an embodiment of the invention in'which the metal base i,the insulation 9 and the metal parts 5 below-the switch lever l in thefirst embodiment are replaced by a block 25 of insulating material suchas steatite. Short posts 26, 26', fixed into the base block 25, clampthe switch parts to the base. The terminals (not shown in Figure l) butactually carried on the outside of the case are, in the embodiment shownin Figures 4 and 5, disposed within the cover as shown at 28, 28, accessbeing obtained to the terminals through suitably placed holes in theouter enclosing cover.

Rod 3 has mounted at its upper end a collar i3 adapted to move within achamber as in a rigid lever l6. Chamber has contact risers BB at itslower end which are contacted by shoulder l9 during downward movement ofrod 3 in response to temperature increase. Rigid lever it is iulcrumedat point on contact plate 8 and has a pip i! at its lower side to impartpressure to switch lever '5. Rigid lever E5 is of electrical insulatingmaterial to avoid making permanent electrical connection between contactplate 8 and resilient switch lever 1. The effective fulcrum of thespring switch lever l is at a point 7' where lines drawn as extensionsto slots 21, 27 and shown dotted in Fig 3 intersect the centre line ofthe lever, taken through the contact M. The point of application ofpressure under pip H is, therefore, between the contactsv and thefulcrum and acts to open the contacts with increasing temperature.

At the upper end of rod 3 is attached a knob l8 accessible from theoutside of the thermostat casing to adjust the operating temperature ofthe thermostat. Rotation of knob It causes rotation of rod 3 and,because of the threaded engagement between rod 3 and tube 2, a relativelongitudinal movement therebetween. By rotating knob I8 in suchdirection as to cause movement of rod 3 and hence shoulder 59 downwardlytoward the switch pack, the temperature at which the switch opens isdecreased, and by rotating knob H3 in the opposite direction thetemperature at which the switch opens is increased.

Rising temperature on the stem tube 2 gives gradual increase of thelength of this tube, which by its screw engagement takes with it rod 3.This movement transmitted to rigid lever Hi from pressure collar I9gives gradual movement to that part of the resilient lever immediatelybeneath the pip ll, this movement causing force to be generated in theresilient'lever in opposition to the resultant magnetic pull on the twinarmature unit l2, l3.

When this force exceeds this resultant pull,-

contact [4 moves with a snap action away from contact I5, thusinterrupting the circuit through the thermostat.

With the heating means switched off falling temperature on the stem tube2 results in contraction of this tube giving movement of the rod 3 andcollar I 9 away from the base' of the switch, thus reducin the pressureof rigid lever is on the resilient lever until the magnet pull is ableto overcome the spring tension of the resilient lever, and re-closecontacts l4, 15.

In co-pending application Serial No. 718,635, filed December 27, 1946,there is disclosed and claimed a somewhat similar magnetic snap actionswitch in which the armature-carrying lever is non-resilient and isactuated by a resilient member.

We claim:

1. An electric thermostat comprising a base, a clamping post extendingfrom the base, a first rigid plate and a resilient lever clamped on saidpost in spaced superposed relation, a permanent magnet fixed to saidplate, an armature assembly including a pair of spaced armatures mountedon said lever and disposed in fixed relation to each other respectivelyon opposite sides of said permanent magnet and spaced therefrom, asecond rigid plate clamped on said post on the side of said leveropposite said first rigid plate, a rigid lever having a point at one endbearing on said second plate and a point at its other end bearing onsaid resilient lever, a contact fixed to said second rigid plate, acontact actuated by the resilient lever and adapted to engage thecontact fixed to said second plate, an actuating member engaging saidrigid lever intermediate said points and adapted to move said rigidlever to actuate said resilient lever and the contact actuated thereby,and means for limiting the movement of the armature assembly withrespect to the permanent magnet so that one ar- 4 mature remains closerto the magnet than the other armature in both the closed and open positions of the contacts, whereby said magnet continuously applies a forcein one direction to the armature assembly which in turn maintains atension between the resilient lever and the rigid lever in opposition tosaid actuating member.

2. An electric thermostat comprising a base, a pair of spaced clampingposts extending from the base, a first rigid plate and a resilient leverclamped on said posts in spaced superposed relation, a permanent magnetfixed to said plate, an armature assembly including a pair of spacedarmatures mounted on said lever and disposed in fixed relation to eachother respectively on 0pposite sides of said permanent magnet and spacedtherefrom, a second rigid plate clamped on said posts on the side ofsaid lever opposite said first rigid plate, a rigid lever having a pointat one end bearing on said second plate and a point at its other endbearing on said resilient lever, a

contact fixed to said second rigid plate, a con-.

tact actuated by the resilient lever and adapted to engage the contactfixed to said second plate,

a member responsive to a thermostat engaging said rigid leverintermediate said points and adapted to move said rigid lever to actuatesaid resilient lever and the contact actuated thereby,

and means including an adjustable stop carried by said first plate forlimiting the movement of the armature assembly with respect to thepermanent magnet so that on armature remainsv closer to the magnet thanthe other armature in both the closed and open positions of thecontacts, whereby said magnet continuously applies a force in onedirection to the armature assembly which in turn maintains a tensionbetween the resilient lever and the rigid lever in opposition to saidactuating member.

3. An electric thermostat comprising a magnet, a movable resilientlever, a pair of armatures carried by said resilient lever for movementtherewith and disposed on opposite sides of said magnet at unequaldistances therefrom in switchclosed position, a fixed contact, acooperating contact carried by said resilient lever, a, second leverfulcrumed at one point fixed with respect to said contact and at anotherpoint on said resilient lever, a thermostatically-controlled memberpositioned to engage said second lever,

between said fulcrum points and to cause movement of said second leverto move said resilient lever in response to change in temperature,andmeans for so limiting the movement of said resilient lever inswitch-opening direction that the one of said armatures which is thenearer to ,the'

magnet in switch-closed position remains the nearer to the magnet in allpositions of the resilient lever.

THEODORE NICHOLAS WILLIAM HARRY WILSON, ARTHUR JOHN CHINNL REFERENCESCITED The following references are of record in the file of this patent:

UNITED STATES PATENTS

